Solar Power - basic calculations

Stand-alone solar Photovoltaic system;

PV installations not connected to a utility power line or grid are
referred to as "stand-alone systems." This comprises the majority of PV systems in operation today. The two basic types of stand-alone systems are: (1)direct systems, which utilize the PV electricity as it is produced, and (2)battery storage systems, which have the capability of storing PV generated electrical energy for use when the sun is not shining.

The information below is for stand alone battery storage PV power
configuration;

A basic solar power system consists of-

Solar Panels: These are the most important and costly
component for a solar power systems. Due consideration should be given while deciding upon the size of the photovoltaic
modules (PVM) or panels depending upon your power requirement.

PVM’s are typically rated by their peak power output when exposed to a solar radiation of 1000 watts per square meter (317.2 BTU/hr-ft
^²) at a module temperature of 25ºC,generally known as peak sun conditions.

Buy PVM’s from reputed manufacturers, most good manufacturers of
Solar PV panels give a guarantee period of 15- 25 years on its solar panels.

Power Inverters:

It is one of the primary components of any independent power
system which requires AC power. Its function is to convert stored DC
power into AC power (220v, 50 cps/Hz)

TRUE SINE WAVE power inverters are recommended over square wave or quasi sine wave inverters. True sine wave inverters cost you more upfront but pay for itself in form of high efficiency, extended battery life and safety of appliances powered by it.

There has been rapid improvement in inverter design &
efficiency manufactured today than was 10-15 years back. A true sine
wave inverter gives AC identical to, or better than, grid power. They have low loss during DC-AC conversion;extend battery life and cuts down maintenance cost in the long run.

It also does not produce any humming in inductive loads, and can be safely used to power computers, printers or any digital equipment without any apprehensions.

Batteries:

They are the component which store energy & discharge it
as needed. For solar energy, DEEP CYCLE BATTERIES are used as they are designed to be discharged over a long period of time, like 100-150 hours, &recharged thousands of times, unlike car batteries which are designed to supply large amount of current for a short interval of time.

It is very important to note that the batteries should NOT
be discharged beyond 50% of their capacity as this will significantly reduce the life of batteries.

Solar Regulators:

These are used to ensure that the batteries receive regulated current from solar panels. Reflected sunlight & temperature can increase the rated output of the solar panel by as much as 20% which can damage the batteries,hence regulators having 20%-30% extra current handling capability should be used in practice.

Regulators also prevent self discharge or back discharge to
the solar panels.

Apart from these, accessories like racking,wiring, AC and DC safety breakers, solar energy monitor, net meter etc may be needed.

To calculate your power requirements;

First,list out all the appliances that will be drawing power from the solar power system along with their power rating;

For example, if one were to use

6 CFL’s (compact fluorescentlight) of 20 watt power rating for 10hours per day,

4 large ceiling fans of 125 watt power rating for 6 hours per day &

1 Fridge of average 250watt power rating for 24 hours (24x7) , the power requirement would be

CFL : 6 x20 watts for (x)10 hours = 1200watt hour (WH)

Fan: 4 x125 watts for(x)6 hours = 3000 WH

Fridge (refrigerator):1 x 250 x 24 = 6000 watts per day

Inverter rating:

For one hour, the total power needed will be

CFL/hr + Fan/hr+ fridge/hr = 120+500+250 = 870 watts.

Providing some fudge factor for conversion loss, we can choose –

Continuous power sine wave inverter, rated 1000 watts

Recommended inverter would be 1500 watt continuous power sine wave inverter.

Number of Solar panels :

Total daily usage = 1200 +3000+6000 = 10200 WH

In Bihar we have an average of 5.5 hours of sunlight/day

for efficient charging (charge hours),

Hence, for a load of 10200 WH per day,it is required that the solar panels should be able to get this in 5.5 hours.

Or. in one hour ; 10200/5.5 = 1854.54 watts,

Adding 20% for conversion losses, we have 2225.45 W

So, for the given load, we need solar panels to harness 2225.45 W per hour.

If we use 75 W PV panels, number of PV

panels needed will be 2225.45/75 = 29.6, say 30 panels

Hence, we would need 30 no’s of 75 watt PV panels.

If we use 150 W PV panels, number of PV

panels needed will be 2225.45/150 = 14.83, say 15 panels

So on & so forth.

Number of batteries:

If we use 75 watt solar panel, which generally produce 4.5 Amps,

We have 30(no. of solar panels) x 4.5 A x 5.5 (charge hours) hr = 726 Ah

Let’s say we want to use 12 V, 150 Ah batteries;

Suggested design discharge (max) for these batteries is 50%, hence we will use only 50% of 150 Ah,i.e 75 Ah

No. of batteries needed = 726/75 =9.68, say 10 batteries

Hence, we would need 10 batteries of 12V, 150 Ah.

These calculations are based on requirements for a totally off grid (standalone) solar PV based power system.

If one had additional source of electrical power, like power from state electric power grid, or a generator set, the number of solar PV panels & number of batteries in battery bank can be significantly reduced by as much as 75 % of design calculations.For that, one would need to know the time (hrs) per day of average regular grid supply in the area, extent of voltage fluctuations and the design output of the generator (if used)

Installation of solar panels:

Tilt data: Positioning of PVM’s (For Patna)

Latitude = 25 Degrees North (Patna,India)

Month	Sun Altitude	Array Tilt	Array Points to:
JAN	45	45	South
FEB	54	36	South
MAR	65	25	South
APR	77	13	South
MAY	85	5	South
JUN	88	2	South
JUL	85	5	South
AUG	77	13	South
SEP	65	25	South
OCT	53	37	South
NOV	45	45	South
DEC	42	48	South

Data is rounded to a whole number and calculated at solar noon on the 21st day of each month.

This data is helpful to get maximum insolation for a given period.
One can also use sun trackers which follow the follow the sun path,hence maximizing insolation.

Although Sun trackers are costly, but if one is interested, the same can be assembled using local materials at a fraction of commercial sun-trackers cost.